Field of the Invention
The invention relates generally to motors, and more particularly to means for controlling a permanent magnet synchronous motor in a manner that eliminates the need to filter a PWM voltage at the output of a drive system for the motor.
Related Art
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. Artificial lift systems may utilize various different types of pumps to lift the oil out of the well. For instance, some conventional installations use rod lift systems in which a surface motor moves a sucker rod up and down in a reciprocating motion to drive a linear pump in the well. Other installations use electric submersible pumps (ESP's) in which a motor and a pump are positioned downhole in the well to pump oil out of the well. ESP's may use motors that have rotary or linear designs, and they may be permanent-magnet or induction motors. These motors are normally powered by electric drive systems that are positioned at the surface of the well.
In one type of ESP, a permanent magnet synchronous motor is used to drive the ESP's pump. A drive system that uses field oriented control techniques is used to provide power to the motor. A field oriented control system controls the voltages that are output by the drive and are provided to the motor (thereby indirectly controlling the currents in the motor's stator). The output voltages are controlled based on a transformation of the time dependent three phase output currents into a time invariant vector in what is referred to as the d-q plane (where d represents a flux component of the vector and q represents a torque component).
Typically, a field oriented control system for a permanent magnet synchronous motor (either rotary or linear) generates some form of PWM voltage outputs. Variable speed drives (VSD's) that generate PWM output voltages to drive deep well ESP's normally require filters at their outputs to remove the high frequency components of the PWM voltages which are caused by the high frequency switching of the IGBTs (insulated gate bipolar transistors) in the drive's inverter. This is necessary because of the length of the power cable that carries the output voltages to the ESP motor.
One of the problems with this type of control system is that the need to include the PWM output filters increases the cost of the system. Additionally, the high frequency switching of the IGBTs that are used in the inverter section of the drive generate losses in the form of heat. This will require extra cooling within the drive enclosure, which will further increase the cost of the system.
It would therefore be desirable to provide systems and methods for controlling the ESP motor that do not require the output filters or extra cooling that is necessitated by the generation of the PWM output voltage.